Nonwoven fabrics and numerous uses thereof are well known to those skilled in the textiles art. Such fabrics can be prepared by forming a web of continuous filament and/or staple fibers and bonding the fibers at points of fiber-to-fiber contact to provide a fabric of requisite strength. The term "bonded nonwoven fabric" is used herein to denote nonwoven fabrics wherein a major portion of the fiber-to-fiber bonding referred to is adhesive bonding accomplished via incorporation of adhesives in the web to "glue" fibers together or autogenous bonding such as obtained by heating the web or by the use of liquid or gaseous bonding agents (usually in conjunction with heating) to render the fibers cohesive. In effecting such bonding, particularly autogenous bonding, the web may be subjected to mechanical compression to facilitate obtaining adequate bonding.
Spunbonded nonwoven fabrics formed of nylon 6,6 are widely used commercially for a number of purposes. Such fabrics exhibit excellent strength and permeability properties and accordingly are desirable for use in construction fabrics, filtration material, and furniture and bedding backing materials.
The fabrics are produced via the well-known spunbonding process in which molten nylon 6,6 is extruded into filaments, and the filaments are attenuated and drawn pneumatically and deposited onto a collection surface to form a web. The filaments are bonded together to produce a strong, coherent fabric. Filament bonding is typically accomplished either thermally or chemically, i.e., autogenously. Thermal bonding is accomplished by passing the web of filaments between the nip of a pair of cooperating heating calender rolls. In autogenous bonding, the web of filaments is transported to a chemical bonding station or "gas house" which exposes the filaments to an activating agent (i.e., HCl) and water vapor. Water vapor enhances the penetration of the HCl into the filaments and causes them to become tacky and thus amenable to bonding. Upon leaving the bonding station, the web passes between rolls which compact and bond the web. Adequate bonding is necessary to minimize fabric fuzzing (i.e., the presence of unbonded filaments) and to impart good strength properties to the fabric. Autogenous bonding has been especially used in forming spunbonded nylon 6,6 industrial fabrics.
Nonwoven fabrics which are strongly bonded overall (for example, by uniform compression of the entire web in the presence of heat and/or appropriate bonding agents) tend to be stiff and boardy and are frequently more similar to paper than to woven textile fabrics. In order to obtain softer nonwoven fabrics more closely simulating woven fabrics, nonwoven "point-bonded" fabrics have been prepared by processes which tend to limit bonding to spaced, discrete areas or points. This is accomplished by application or activation of an adhesive or bonding agent and/or application of heat and/or pressure at the points where bonding is desired. For example, the web to be bonded can be compressed between a pair of rolls or platens, at least one of which carries bosses or a land and groove design sized and spaced to compress the web at the desired points. The compression device can be heated to effect thermal bonding of the web fibers or to activate a bonding agent applied to the web. In the actual practice of preparing point-bonded fabrics, however, it is frequently difficult or even impossible to limit bonding to the desired points. In many processes, web areas between the desired bond points are subjected to sufficient heat, compression, activated bonding agent, or adhesive to effect "tack" bonding of fibers outside the desired bond points. Such tack bonding is believed to contribute significantly to undesired fabric stiffness.
It has been found that most point-bonded nonwoven fabrics, particularly those having a large number of tack bonds, and many overall bonded nonwoven fabrics can be significantly softened by subjecting the fabric to mechanical stress. For example, the fabric can be washed in conventional domestic washing machines, drawn under tension over a sharply angled surface such as a knife blade, stretched, twisted, crumpled, or subjected to various combinations of such treatments. Such treatments are believed to effect softening primarily by breaking weaker fiber-to-fiber bonds such as tack bonds which can be broken without breaking the point- or intentionally-bonded fibers. These methods are relatively effective but subject to certain practical problems. For example, drawing a nonwoven fabric over a knife blade with sufficient force to effect substantial softening frequently results in an undesirably high level of physical damage to the fabric. Washing of nonwoven fabrics generally yields good results, but is a batch operation not typically adaptable for use in continuous processes of the type employed commercially for production of nonwoven fabrics.
Another method for softening nonwoven fabrics is by impinging the fabric with a fluid jet. This is, however, an additional and potentially cumbersome production step, resulting in increased manufacturing costs.
It is apparent that a commercially practical process for a simpler, more cost-effective method for the softening of nonwoven fabrics would satisfy a long-felt need in the nonwoven textile art.